Direct-current to direct-current (DC-DC) converters can be implemented using inductors or capacitors as the energy storage devices. Switched inductor (SL) DC-DC converters use a chopper circuit to generate a square voltage signal from the input battery DC voltage. An output inductive filter is used to extract the DC component of the square signal. Thus, the voltage conversion ratio from the input battery to the supplied circuit can be continuously controlled through the duty cycle of the square voltage signal. On the other hand, switched capacitor (SC) DC-DC converters utilize different topologies of capacitors to provide discrete voltage conversion ratios.
As opposed to SL voltage converters, SC voltage converters suffer from fixed voltage conversion ratio, m:n, from the input to the output terminals. Indeed, SC converters can only deliver output voltages with high efficiency at discrete ratios of the input voltage. In order to obtain continuous voltage regulation under line and load variations, the SC equivalent output resistance is modulated, through the switching frequency, and hence the SC is essentially operated as a linear regulator. Therefore, the SC efficiency degrades severely as the desired output level deviates from the SC unloaded voltage level.
The intuitive method to solve such problem in SC DC-DC converters is to change the unloaded conversion ratio, m:n, to obtain the desired output voltage, where the voltage drop across the converter's output resistance is minimized. However, large number of conversion ratios substantially increases the number of components and eventually the converter's complexity. Therefore, the conversion ratio is only changed when the output falls substantially below the unloaded conversion ratio, m:n, such that the linear regulation through the output resistance is limited and efficiency is kept within a reasonable range.